Heat-treatment of magnesium base alloys



Patented Aug. 29, 1950 HEAT-TREATMENT OF MAGNESIUM BASE ALLOYS Charles J. P. Ball, Alfred Claude Jessup, and Philip Andrew Fisher, Clifton Junction, near Manchester, England, assignors to Magnesium Elektron Limited, Clifton Junction, near Manchester, England, a British company No Drawing. Application January 4, 1949, Serial No. 69,242. In Great Britain January 6, 1948 2 Claims.

This invention relates to the heat treatment of magnesium base alloys of the kind containing at least 90% magnesium, 443% zinc, and between 0.5% and 1% of zirconium in a condition which (together with the magnesium which contains it) is readily soluble in an aqueous solution of hydrochloric acid consisting of 30 cos. of H01 (specific gravity 1.16) to 85 cos. Of water, sufficient acid being added during dissolution to maintain the initial concentration, with or without cadmium up to 5% and/or quite small quantities or traces of one or more of the following via, rare earth metals, silver, thorium, mercury, beryllium, copper, bismuth, barium, lead and calcium. The maximum quantity of rare earth metals, silver, thorium, copper, bismuth and barium, will be 0.5% each and in total; of mercury and lead 1%; and of beryllium maximum solubility; and calcium 0.2%. The alloys of the present invention must, in accordance with the invention described in the specification of the applicant companys British Patent No. 511,137, .be substantially free from elements which precipitate zirconium'from a magnesium melt, such as aluminium manganese, silicon, tin, cobalt, nickel and antimony.

It has been found that the mechanical properties of alloys of the kind above referred to can be improved by heat treatment at temperatures which result in the precipitation of the zirconium and/or zinc in a finely divided condition. We have found that particularly good results can be obtained by carrying out this heat treatment at temperatures of about l80-200 C. If it is desired to effect a stress relief annealing of castings made from these alloys, the heat treatment may often be carried out at a temperature of at least 300-330 C. However, it isto be expected and has been found that a heat treatment at temperatures of 300330 C. would result in a reduction in mechanical properties presumably due to the precipitation of the zirconium in a coarse form.

We have now made the surprising discovery that a heat treatment of such alloys at temperatures of about 300350 C. for comparatively short periods followed by a heat treatment at a considerably lower temperature produces appreciably higher mechanical properties than are obtainable by a heat treatment at the lower temperature alone, probably by avoiding the precipitation of the zirconium in a coarse condition. This shortterm heat treatment at BOO-350 C. also effects adequate stress relief of the casting.

The coarse condition of zirconium precipitation is easily observable in a micro-Photograph when the heat treatment is sufiiciently prolonged but is not so easily distinguishable in the early stages of precipitation. Nevertheless even these early stages of precipitation will result in loss of mechanical properties in the fully heat treated casting. Similarly the finely divided precipitation can also be more definitely measured by the final mechanical properties. Therefore for the purpose of the present invention the periods and temperatures of the two parts of the heat treatment ar selected in order to avoid loss of proof stress, or more accurately to ensure obtaining at least the following minimum figures viz., 10 tons per square inch 0.1% proof stress for an alloy containing 4% zinc and proportionately up to 11.0 tons per square inch for an alloy containing 6% zinc. A few quite simple tests are therefore sufIi-cient to indicate the required treatment according to the present invention for any particular alloy. Thus if an alloy be subjected to the two treatments and the final proof stress is below that above quoted, it can be assumed that either (a) the first part of the treatment is too long or too short and;or (b) the second part of the treatment is too short. Therefore a further test is made with a shortened or lengthened period for the first part of the treatment and/or a more extended treatment for the second part of th treatment, being guided by the fact that in the first part of the treatment a period more than 3 hours at 400 C. is generally too long, and more than six hours at 330 C. is generally too long, and more than about 12 hours at 260 C. is similarly undesirable, and in the second part of the treatment a period of at least 6 hours is required at 240 C. and at least 48 hours at 150 C. and of course longer at C. No loss of proof stress occurs by extending the period of the second part of the treatment.

The proof stress figures above quoted are for sand cast test bars made in accordance with Directorate of Technical Development Specification No. 58B, the proof stress being determined from a curve drawn by plotting extensometer readings against a continuously increasing loading applied to the test bar.

It is to be understood that in applying the invention to castings other than test bars the appropriate times and temperatures for the alloy can be ascertained for test bars and then the same times and temperatures applied to the castmgs.

Another method of selecting the appropriate periods of treatment is based on a comparison of the proof stress obtained by the double treat- 3 inent according to this invention and the proof stress obtained by a treatment of the same alloy at 180 C. for 24 hours. Thus if the double treatment gives a proof stress greater'than that given by the low temperature treatment alone, the double treatment may be considered to be satisfactory. For example a double treaement of 2 hoursat 330 C. and 24 hours at 180 C. may give 10.5 tons per square inch proof stress as compared with an average of about 9.9 tons per square inch proof stress after only 24 hours at 180 C. If a double treatment gives a less proof. stress than the low temperature treatment alone, then the periods and temperatures must be readjusted on the lines indicated above i. e. avoidingto'o'long or too short periods for the first part of the treatpart of the treatment, and in either part of the treatment lengthening the period is equivalent to increasing the temperature.

According to the present invention, therefore, the alloys are subjected to a heat treatment at a temperature above 260 C. (generally above 300 C.) but below 400 Cland the alloy is subsequently subjected to a further heat treatment at a temperature between 100 C. and 240 C., the temperature and period of both these treatments being selected so as to produce one or both of the following results:

1. Then applied to a test bar according to Directorate of Technical Development SpecificationNo. 59B (1. e. a bar cast in a dry sand mold inclined at an angle approximately from vertical and the temperature of the :.mold prior to casting being between 10 and 40 C.), this test bar will have a 0.1% proof stress of at least 10 tons per square inch at 4% zinc, this figure rising proportionally with increasing zinc content to 11.0 tons per square inch at 6% zinc.

2. A 0.1% proof stress greater than the 0.1% proof stress produced by subjecting the alloy to a temperature of 180 C. for 24 hours.

The treatment at the higher temperature does not result in any appreciable solution of any zinc-rich beta phase which may be present.

The heat treatment at the higher temperature is preferably carried out for a period of about 1-12 hours dependent upon the temperature as above indicated, the lower temperature generally requiring a longer duration of treatment. The treatment of the alloys for 2 hours at 330 C. has given particularly good results while achieving the complete stress relief commonly required. The treatment at the low temperature may be effected for a period of 6-48 hours at temperatures of 240-150 C., or even down to 100 C. for a period of about .72 hours, the lower temperature again generally involving the longer duration of treatment. A treatment at 180 C. for 24 hours has given particularly good results. The treatment at the'lower temperatures may nowever be continued for a number of days.

The advantage achieved by the present invention is illustrated by way of example in the following experimental results obtained by heat treatment of an alloy of the following composition:

Per cent Zirconium (soluble in hydrochloric acid as hereinbefore described) 0.7 Zinc 4.5

Magnesium remainder Ultimate Timeand' Temperature of g g Tensile g g g I Heat Treatment tons/sq. in. g f g cent on 2 As Cast 7. 9 16. 0 9. s 24 hrs. at 180 0 9. 9 17. 7 9. 0 24 hrs. at 330 C 8.6 16.4 9. 5 2 hrs. at 330 0 9. 3 17. 7 13. 5 2 hrs. at 330 O. 24 hrs. at

180 0 10.5 18.5 10. 0 24 hrs. at 330 C. 24 hrs. at

We claim: 7

1. In amethod of the character describecl'improving the properties of castings of a magnesium base alloy which contains at least magnesium, between 4% and 6% zinc, and between.0.5% and 1% .of zirconium, by, first, subjecting said alloy to a heat treatment at a temperature between 260 C. and 400 C. for a period of time of at least one hour and not exceeding three hours at 400 C. proportionally to a period of time not exceeding twelve hours at 260 C-., and, second, subjecting the so-treated alloy to a further heat treatment at a temperature between C. and 240 C. for at least six hours.

2. In a method of the character described, improving the properties of castings of a magnes'ium' base alloy which contains at least 90% magnesium, between 4% and 6% zinc, andbetween 05% and 1% of zirconium, by, first, subjecting said alloy to a heat treatment at a temperaturebetween 260 C. and 400 C. for a period of time of at least one hour and not exceeding three hours at 400 C. proportionally to a period.

of time not exceeding twelve hours at 260 C., and, second, subjecting the so-treated alloy to a further heat treatment at a temperature between 100 C. and 240 C. for a period of at least six hours at 240 C., proportionally to a period of at least 72 hours at 100 C.

CHARLES J. P. BALL.

ALFRED CLAUDE JE'SSUP.

PHILIP ANDREW FISIER.

REFERENCES CITED The following references are of record inth file of thispatent:

UNITED STATES PATENTS Number Name Date 1,936,550 Gann et a1. Nov. 21, 1933 FOREIGN PATENTS Number Country Date 815,988 France July 27, 1937 211,152 Switzerland Aug. 31, 1940 

2. IN A METHOD OF THE CHARACTER DESCRIBED, IMPROVING THE PROPERTIES OF CASTINGS OF A MAGNESIUM BASE ALLOY WHICH CONTAINS AT LEAST 90% MAGNESIUM, BETWEEN 4% AND 6% ZINC, AND BETWEEN 0.5% AND 1% OF ZIRCONIUM, BY, FIRST, SUBJECTING SAID ALLOY TO A HEAT TREATMENT AT A TEMPERATURE BETWEEN 260*C. AND 400*C. FOR A PERIOD OF TIME OF AT LEAST ONE HOUR AND NOT EXCEEDING THREE HOURS AT 400*C. PROPORTIONALLY TO A PERIOD OF TIME NOT EXCEEDING TWELVE HOURS AT 260*C., AND, SECOND, SUBJECTING THE SO-TREATED ALLOY TO A FURTHER HEAT TREATMENT AT A TEMPERATURE BETWEEN 100*C. AND 240*C. FOR A PERIOD OF AT LEAST SIX HOUES AT 240*C., PROPORTIONALLY TO A PERIOD OF AT LEAST 72 HOURS AT 100*C. 